The fibrillation of fibers, fibrillated fibers and their uses are well-known to those skilled in the art. For example, U.S. Pat. No. 2,810,646 to Wooding et al discloses a water laid web comprising filtered, heat-bonded, water-fibrillated, wet-spun filaments. The filaments are of a polymer selected from the group consisting of polymerized acrylonitrile and a copolymerized mixture of acrylonitrile and up to 15%, by weight, of at least one other monomer copolymerizable therewith. U.S. Pat. No. 4,495,030 to Giglia discloses the use of a fibrillated fiber to provide cohesiveness and support to a wet-laid sheet containing active carbon and submicron glass fibers. U.S. Pat. No. 4,565,727, also to Giglia, discloses the use of a fibrillated fiber to provide cohesiveness and support to a wet-laid sheet containing active carbon in the form of carbon fibers and carbon particles.
Various nonwoven structures using a fibrillated acrylic fiber were disclosed in Giglia et al; Novel Nonwoven Activated Carbon Fiber Papers presented to a meeting of the American Chemical Society in April of 1984.
Recently, there has been much interest in the possible use of nonwoven fabric technology to produce paper and felt like structures containing activated carbon for use in chemical protective clothing and filtering applications including both gas and liquid filtering. The aforementioned Giglia paper described several nonwoven adsorptive felt like structures having loadings of activated carbon fibers or powders. In that paper it was disclosed that a fibrillated acrylic fiber, produced according to the process set forth therein, was useful in permitting high loadings of filler materials, such as activated carbon fibers and powders in the nonwoven fabric while maintaining good wet strength and chemical resistance.
While many binding agents have been available in the past, fibrillated fibers are becoming of interest as they provide fine diameter fibrils as opposed to those of heavier spun fibers. Generally, spun fibers are produced in sizes of ten microns or greater while it has been the experience that sizes of less than a micron (cross section) are required to entrap and bind fine particles in nonwoven and other composite structures. Need exists now, however, for binders which provide such entrapment properties which also provide reinforcement and strength to composite constructions. While the fibrillated fibers of the prior art have provided adequate and improved characteristics, recognized needs for further improvement in this field are apparent and a welcome contribution to the art would be a fibrillated fiber having highly desired physical characteristics of low Canadian Standard Freeness in combination with relatively high Tensile Strength. Heretofore, the limits of these properties in the area of acrylic fibers has been such that fibrillated acrylic fibers have not been available with a Canadian Standard Freeness below about 200 and certainly not available in combination with a useful Tensile Strength such that the material could be processed on conventional nonwoven fabric lines. These and other shortcomings of the prior art have been remedied by the discovery of the instant invention which will be described herein as follows.